Hydrogen peroxide, homolytic decomposition

Only a limited number of peroxides are suitable for crosslinking purposes. The most suitable are those which form radicals through homolytic decomposition. These radicals are so aggressive that they can split hydrogen atoms from the polymer chain, producing stable peroxide decomposition products and polymer radicals. These polymer radicals can then be recombined, producing stable C-C crosslink bonds. 

A wide variety of peroxides have been used to produce alkyl radicals, either directly as fragments of the decomposition of peroxides, or indirectly by hydrogen abstraction from suitable solvents. The production of alkyl radicals used in homolytic alkylation has been accomplished by thermal or photochemical homolysis and recently also by redox reactions due to the possibilities offered by alkylation in acidic aqueous solution. 

Homolytic Decomposition of Hydrogen Peroxide by Action of Hydrogen Ions... 

Similar results were obtained for tert-butyl hydroperoxide and perchloric acid in 2-propanol. Thus, it is evident from the decomposition of hydrogen peroxide into free radicals that both heterolytic and homolytic reactions may be catalyzed by hydrogen ions. Further research is needed to investigate proton catalysis in certain homolytic reactions. 

Some radical initiators decomposing by heat can also be decomposed by UV radiation. Photopolymerization of acrylonitrile in the presence of AIBN or hydrogen peroxide, or of other initiators [75-78] has been reported. The quantum yield of AIBN decomposition is 0.4 at 298 K and 0.6 at 318 K. Photopolymerization of methyl methacrylate, styrene, and vinyl acetate can be initiated by tetramethylsilane, methylchlorosilanes, and halides of Group IV metals [79]. We assume that the radicals are formed by homolytic splitting of the covalent bond... 

Thermal or Photochemical Decomposition of Hydrogen Peroxide Hydrogen peroxide decomposition can occur by homolytic or heterolytic scission... 

Elements of Group III and the n-butyl analogue, have been obtained by treating the dialkoxy-chloroborane with hydrogen peroxide in diethyl ether.216 A mechanism for the thermal decomposition to boric acid and B(OR)3 is postulated in which the first stage is homolytic cleavage of the O—O bond. 

The oxidation of cyclohexene using hydrogen peroxide was chosen as a test reaction for the catalytic evaluation of the titanium modified hexagonal NaY sanq)les. Scheme I illustrates some of the typical products of cyclohexene oxidation. The epoxide and the diol which is a hydrolysis product of the epoxide, generally reflect a concerted process. In contrast the allylic alcohol and ketone are often ascribed to an autoxidation or radical process. We anticipated that some homolytic decomposition of the peroxide may be observed with these acidic zeolites. In fact, there was -74% conversion of H2O2 over calcined hexagonal NaY after heating at 55 C for 24 hours. This resulted in only a 1% conversion of... 

The hydroxyl radicals may be formed by homolytic cleavage of hydrogen peroxide (Equation 12.1) or by metal ion catalyzed decomposition (Equations 12.2 and 12.3)... 

Radicalar polymerizations are very commonly used. In this case, the active site is an atom bearing a unpaired electron (free bond). The initiator is an organic material which can spontaneously split by homolytic breaking and in this way can produce radicals able to attack the monomer and thus to initiate the process. Actually, as initiators, one uses peroxides (R -O-O-R"), hydroperoxides (R -O-OH) (for instance hydrogen peroxide H202, or cumene hydroperoxide) and aliphatic azoics (R -N=R"). For each initiator, there is a domain of temperature often narrow (about 10 degrees) in which the homolytic decomposition occurs with an acceptable velocity. 

The competitive method employed for determining relative rates of substitution in homolytic phenylation cannot be applied for methylation because of the high reactivity of the primary reaction products toward free methyl radicals. Szwarc and his co-workers, however, developed a technique for measuring the relative rates of addition of methyl radicals to aromatic and heteroaromatic systems. - In the decomposition of acetyl peroxide in isooctane the most important reaction is the formation of methane by the abstraction of hydrogen atoms from the solvent by methyl radicals. When an aromatic compound is added to this system it competes with the solvent for methyl radicals, Eqs, (28) and (29). Reaction (28) results in a decrease in the amount... 

Thermal decomposition in three different ways, i.e. homolytic, polar and radical induced decomposition, as well as intermolecular reaction of sulfonyl peroxides are the main reactions displayed by sulfonyl peroxides. When bis(arylsulfonyl) peroxides are allowed to decompose at 25-40 °C in chloroform, homolytic 0—0 bond fission followed by hydrogen abstraction from the solvent results in the formation of the corresponding arylsnlfonic acids. Mixed acyl sulfonyl peroxides undergo complicated thermal decomposition in solution, and have been used commercially as polymerization initiators, since they provide a source of free radicals at a relatively low temperature . 

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